There is much interest at the present time in the practical realization of monolithic optical circuits. Such circuits as presently contemplated for use in optical communications systems will not only have light sources and detectors but they will also have means of routing optical signals between the individual components on each chip as well as means of modulating the optical signals. The signals may be modulated in, for example, their intensity, amplitude, phase, or polarization. Monolithic optical circuits are of interest in optical communications as, for example, integrated optical repeaters. Such repeaters would permit optical signals to be detected, regenerated, and transmitted in a single integrated device.
The modulated output optical signal from such an integrated device may be obtained by directly modulating the light source in the monolithic optical circuit, i.e., internal modulation, or an external modulator may be used to modulate the light source. Both approaches to signal modulation are being explored at the present time. However, the latter approach, that is, the use of an external modulator, has the advantage that it theoretically promises to be more reliable at high data rates and permits the light source to be optimized solely for its optical output characteristics. Internal modulation often requires compromises being made in device design so that the device may be efficiently and reliably modulated directly.
As might be expected, many types of modulators have been developed. For example, Journal of Applied Physics, 47, pp. 2069-2078, May 1976, describes a (110) GaAs/Al.sub.x Ga.sub.1-x As p-n junction modulator. This modulator requires nearly degenerate TE and TM modes and this condition was satisfied in the devices described by having a small difference in the refractive index between the cladding and waveguide layers. However, the resulting planar guided modes were only weakly bound and it was therefore difficult to control the lateral guiding and the device capacitance because the initially relatively thick, approximately 2.0 .mu.m, top cladding layer had to be etched to a relatively small thickness, i.e., approximately 0.3 .mu.m, to provide the necessary lateral guiding. The thick cladding layer was necessary to prevent high TM mode losses through the metallic contacts.
Rib waveguide switches are described in Applied Optics, 17, pp. 2548-2555, Aug. 15, 1978, which have MOS electro-optic control. The switches had low losses and were capable of efficiently switching signals between optical channels. Other methods of control, for example, heterojunctions formed by a layer of CdO on the semiconductor, are also possible.
Rib waveguide polarizers are described in Applied Physics Letters, 36, pp. 237-240, Feb. 15, 1980. A metal cladding on top of a thin oxide layer provided differential absorption of the TE and TM modes in the waveguide resulting in polarization of the optical output. The polarizer provides efficient discrimination between TE and TM modes. Consequently, the integrated combination of polarizer devices with polarization modulators leads to direct intensity modulation of the light.